CUCP#50 kicad5: figure what the Margin layer is used for; provide minimalistic
        kicad_pcb+gerber examples accordingly.

#50 margin layer: (answer by Karl Hammar)

 kicad-doc/build_5.1.0/src/pcbnew/en/pcbnew.pdf p. 40/140:

 5.3.3 Independant Technical Layers

 Edge.Cuts
   This layer is reserved for the drawing of circuit board
   outline. Any element (graphic, texts. . . ) placed on this layer
   appears on all the other layers. Use this layer only to draw board
   outlines.

 Margin
       Board's edge setback outline (?).

So the kicad folks doesn't know to explain it.

You can draw tracks outside the Margin, but not outside the Edge.Cuts.
You can place components outside the Edge.Cuts.

So I guess it is for the designers own information only.